In a continuous annealing furnace for annealing steel strips, for example, in the startup of the furnace after the furnace is opened in the air or in the case where the air enters the atmosphere in the furnace, to reduce the water content and oxygen concentration in the furnace, the following method is conventionally widely performed: the temperature in the furnace is increased to evaporate water in the furnace and a non-oxidizing gas such as an inert gas is subsequently supplied into the furnace as a gas that replaces the atmosphere in the furnace and the gas in the furnace is simultaneously removed to thereby replace the atmosphere in the furnace with the non-oxidizing gas.
However, this conventional method requires a long time to decrease the water content and oxygen concentration of the atmosphere in the furnace to certain levels suitable for the normal operation. Since the furnace cannot be operated until the certain levels are achieved, the productivity is considerably decreased, which is a problem.
In recent years, in the fields of automobiles, household electrical appliances, construction materials, and the like, demands for high-tensile steel that can contribute to, for example, reduction in the weight of structures have been increasing. According to high-tensile steel techniques, when Si is added to steel, high-tensile steel strips having a high hole expansion ratio can be produced; and when steel is made to contain Si or Al, retained γ tends to be formed and steel strips having a high ductility can be provided.
However, when high-strength cold-rolled steel strips contain elements susceptible to oxidation such as Si and Mn, these elements susceptible to oxidation are concentrated in the surfaces of the steel strips during annealing to form oxides of Si, Mn, and the like. As a result, the steel strips have a poor appearance or the steel strips are not sufficiently treated with a chemical conversion treatment such as a phosphate treatment, which is problematic.
In the case of hot dip galvanized steel strips, when the steel strips contain elements susceptible to oxidation such as Si and Mn, these elements susceptible to oxidation are concentrated in the surfaces of the steel strips during annealing to form oxides of Si, Mn, and the like. As a result, the coatability of the steel strips is degraded and uncoated defects are generated; or the alloying rate in the galvannealing after galvanizing is decreased, which is problematic. In particular, as for Si, oxide films composed of SiO2 formed on the surfaces of steel strips considerably degrade the wettability of the steel strips with the hot dip coating metal; and SiO2 oxide films inhibit diffusion between the base metal and the coated metal in the galvannealing. Thus, the problems that the coatability is degraded and the galvannealing is not properly performed tend to be particularly caused.
To avoid these problems, a method of controlling the oxygen potential of the atmosphere in annealing may be employed.
As a method of increasing the oxygen potential, for example, Patent Literature 1 discloses a method of controlling the dew point in a region extending from the latter portion of the heating zone to the soaking zone to be a high dew point of −30° C. or more. This method is advantageous in that it provides a certain level of effects and the dew point is easily controlled to be high on an industrial scale. However, the method has a disadvantage that steels (for example, Ti-based IF steel) for which it is desirable to avoid high dew points are not easily produced. This is because it takes a very long time to make an annealing atmosphere that has been made to have a high dew point have a low dew point. In addition, since this method makes the atmosphere in the furnace be an oxidizing atmosphere, a mistake in the controlling causes a problem that oxide adheres to rolls in the furnace and pickup defects are generated and a problem that the furnace walls are damaged.
Alternatively, a method of making the oxygen potential low may be employed. However, Si, Mn, and the like are very susceptible to oxidation. Accordingly, in large continuous annealing furnaces installed in CGL (continuous galvanizing line) and CAL (continuous annealing line), it has been considered to be very difficult to stably maintain an atmosphere having a low dew point of −40° C. or less in which oxidation of Si, Mn, and the like is suppressed.
Techniques of efficiently obtaining an annealing atmosphere having a low dew point are disclosed in, for example, Patent Literatures 2 and 3. However, these techniques are intended to be used for single-pass vertical furnaces, which are relatively small furnaces. These techniques are not intended to be used for multi-pass vertical furnaces for CGL and CAL and hence the probability that the dew point is not efficiently decreased by the techniques is very high.